This invention relates to detecting the level of a material. More particularly, this invention relates to using the resonant frequency of a tube to detect if a material has reached a specified level.
It is often necessary to determine whether the surface of a material is above or below a particular level. This determination is often referred to as point level sensing. Numerous types of point level sensors are known. With liquids, magnetic floats can be used in conjunction with magnetically operated switches. The switches are located at fixed positions in a tank. When the magnetic float reaches a switch that switch""s state changes.
While floats are useful, their operation depends upon mechanical movement and upon the density or densities of one or more liquids. Floats are generally not suitable for sensing solids (including powders), thick or clinging liquids, and foams. Furthermore, floats are often rather large and are subject to leaks and other failures.
Another type of point level sensor is the vibrating rod. In such sensors a rod element that vibrates at a specific frequency is located at a particular level. When a material comes into contact with the rod element the vibrations either stop or the vibration frequency changes. However, such sensors tend to be relatively large, sensitive to thick, clinging materials, and expensive. Furthermore, with vibrating rod sensors it is difficult to sense light objects, such as fine powders and foams.
Many other types of point level sensing systems are known, including ultrasonic, radiation, thermal conductivity, paddle wheels, and slow-wave time-domain reflectometry. However, some of these systems are expensive, some are inaccurate, some are subject to clogging, others are difficult to use, while others are difficult to manufacture. In any event, soft materials, foams, and materials that cling remain difficult to sense.
Another type of point level sensing uses acoustic waves. For example, U.S. Pat. No. 5,128,656, entitled xe2x80x9cLevel Detecting Method and its Apparatusxe2x80x9d issued on Jul. 7, 1992 to Watanabe teaches a method of point level sensing that uses stationary waves in a tube. According to that patent, a tube having one end free and an acoustic source at the other end is located such that the free end is at the predetermined position. The acoustic source then generates acoustic waves that are capable of producing standing waves having either a node or an antinode at the free end. If a material closes the free end, depending on whether nodes or antinodes are used, standing waves are either produced or extinguished. From the existence or absence of the standing waves a determination is made as to whether the material had reached the predetermined level. In particular, Watanabe teaches determining if standing waves are present by sensing the impedance of the sound source. The acoustic method of U.S. Pat. No. 5,128,656 has the advantage that even very light materials, such as feathers and cotton, can be sensed.
U.S. Pat. No. 5,128,656 is based on the physics of resonant tubes. A tube having an effective length L that is filled with a medium having a speed of sound of c can produce two different sets of resonant frequencies. If the tube is closed at both ends (or open at both ends) the possible resonant frequencies are:
fnc=nc/2L,
where n=1, 2, 3, . . .
If the tube is open at one end and closed at the other end the possible resonant frequencies are:
fno=(2nxe2x88x921)c/4L,
wherein n=1, 2, 3, . . .
While U.S. Pat. No. 5,128,656 may be useful, it may not be optimal. It appears to have drawbacks in that its method of sensing resonance is not particularly easy to implement, it may have operational reliability problems, and it appears to be difficult to use with caustic vapors.
Therefore, a new level sensor based on the physics of resonant tubes would be beneficial.
The principles of the present invention provide for point level sensors that can sense the level of a material. Advantageously, the principles of the present invention can enable level sensors capable of detecting the level of many materials, including very light solids, such as feather, cotton, and powders, and of almost all liquids, including highly viscous liquids that tend to cling.
A level sensor according to the principles of the present invention includes a tube having a sensor end and an acoustic transducer at the other end. A driver circuit drives the acoustic transducer in an attempt to produce a standing wave in the tube. After a time sufficient to produce a standing wave the driver circuit stops driving the acoustic transducer. A sensing network then monitors the decay of the acoustic waves in the tube to determine if a standing wave was produced. Based on that determination, a level signal is produced that indicates whether a material has closed the sensor end of the tube.
According to one embodiment of the present invention, the driver circuit drives the acoustic transducer with a frequency that would produce standing waves if the sensor end is open. If a standing wave is produced, as determined by the acoustic decay in the tube, a level signal indicates that the material has not reached the sensor end.
Beneficially, the driver circuit also drives the acoustic transducer at a frequency that cannot produce a standing wave (resonance). In that case, the presence of a standing wave is determined by comparing the acoustic decay in the tube at the possible resonant frequency against the acoustic decay in the tube at the frequency that cannot produce resonance.
Even more beneficially, the driver circuit also drives the acoustic transducer at a plurality of possible resonant frequencies in an attempt to find a resonant frequency. This enables temperature compensation and provides an easy method of compensating for changing vapor concentrations in the tube.
Also beneficially, the driver circuit drives the acoustic transducer in an attempt to produce a resonant frequency when the sensor end is closed. This enables a fail-safe approach in that the tube must be either open or closed (neglecting a small xe2x80x9ctransition rangexe2x80x9d where the tube is neither open nor closed).
According to another embodiment of the present invention, the driver circuit drives the acoustic transducer in an attempt to produce standing waves when the sensor end is closed. If a standing wave occurs, as determined by acoustic decay in the tube, a level signal indicates that the material being sensed has reached the sensor end.
Beneficially, the driver circuit further drives the acoustic transducer at a frequency that cannot produce a resonance. Then, the presence of a standing wave is determined by comparing the acoustic decay in the tube at the possible resonant frequency with the acoustic decay in the tube at the frequency that cannot produce resonance.
Even more beneficially, the driver circuit also drives the acoustic transducer at a plurality of possible resonant frequencies in an attempt to find a resonant frequency. This enables temperature compensation, compensation of the vapor in the tube, and compensates for a material that rises into the tube.
Alternatively or in addition, the driver circuit drives the acoustic transducer in an attempt to produce a resonant frequency when the sensor end is open. This enables a fail-safe approach in that the tube must be either open or closed (neglecting a small xe2x80x9ctransition rangexe2x80x9d where the tube is neither open nor closed).
The principles of the present invention further provide for a method of determining whether a material being sensed has reached a predetermined level. In such a method a sensor end of a tube is located at the predetermined level. An acoustic frequency attempts to create standing waves in the tube. The acoustic frequency is stopped, and the acoustic decay in the tube is monitored. A determination is then made as to whether standing waves occurred, and, based on that determination, a signal is produced that identifies whether the material being sensed has reached the predetermined level.